1. Field of the Invention
This invention relates generally to electronic devices, and more particularly to semiconductor devices using photon-photon interactions.
2. Description of Related Art
The principal obstacle to the realization of few-photon all-optical devices is a known weakness of photon-photon interaction. Photon-photon interaction can be increased in semiconductor nanostructures by designing large optical nonlinearities, but those are usually achieved at the cost of a strong absorption and sometimes at the cost of speed. Atomic dark resonances were proposed in atomic systems as a means to obtain giant nonlinearities without loss and to achieve single photon blockade, i.e. a system allowing the transit of only a single photon at a time, analogous to a Coulomb blockade. See, for example, H. Schmidt and A. Imamoglu, Opt. Lett 21, 1936 (1996) and A. Imamoglu et al., Phys. Rev. Lett. 79, 1467 (1997), which are incorporated by reference herein.
Unfortunately, a similar scheme cannot be adapted to semiconductor structures, due to the short dephasing times inherent to them. It is however highly desirable to realize photon blockade in semiconductors, as well for device application as for the extreme adaptability of these systems.
Biasing quantum wells (QW) as a means of enhancing optical nonlinearities has already been proposed more than ten years ago. See D. S. Chemla, D. A. B. Miller, and S. Schmitt-Rink, Phys. Rev. Lett. 59, 1018 (1987), M. Yamanishi, Phys. Rev. Lett. 59,1014 (1987), A. Shimizu, Phys. Rev. Lett. 61, 613 (1988), and H. Kuwatsuka and H. Ishikawa, Phys. Rev. B 50, 5323 (1994), which are incorporated by reference herein. The nonlinearity arises from the field screening by the electron-hole dipole induced by the biasing. This process was found, however, to be practically of little use; a gain of nonlinearity arises only at small bias and in narrow QWs, where carrier separation is relatively small. The strongest optical nonlinearity was found for 7 nm wide QWs and with an electric field of 150 kV cmxe2x88x921. In wider QWs, the carrier separation is enhanced, but the gain due to increased screening is injured by the decrease of excitonic oscillator strength.
The situation is very different in exciton polariton microcavities. In the strong coupling regime, which usually prevails in QW microcavities, photon-exciton coupling is saturated and is not injured, in first-order approximation, by a decrease in oscillator strength. The present invention uses wider biased QWs to increase the field screening effect by several orders of magnitude at no detriment to the photon-exciton interaction.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses methods and devices using strong photon-photon interactions. The device comprises an exciton polariton system in a strong coupling regime and excitons with spatially separated electron and hole pairs. The method comprises providing a signal light to an exciton polariton system in a strong coupling regime and excitons with spatially separated electron and hole pairs, providing a control light to the exciton polariton system and removing the control light
Various applications of the invention are available, including optical turnstiles, all-optical switches, all-optical phase retardation, low-power saturable transmitters and mirrors. In addition, the applications may operate at single- or few-photon levels.
Beside the strong photon-photon coupling, particular embodiments of the invention have several additional advantages to conventional exciton systems in QWs. A photon is either transmitted or reflected by the microcavity, but not absorbed. The polaritons, particularly the lower one, can only relax into the cavity mode, thus there is no absorption in the classical meaning of the term; each photon penetrating the cavity will exit. Consequently, there is no need to detune the photon energy from resonance to avoid absorption, as in conventional excitonic systems, and the invention does not suffer from dissipation.
In addition, there is a phenomenon in exciton polariton microcavities called motional narrowing that effectively reduces the polariton inhomogeneous broadening due to interface roughness. As a consequence, the polariton linewidth is narrower than usual exciton linewidth, which farther contributes in reducing the light intensity necessary to switch the microcavity.
An object of the present invention is to provide fast, low-absorption optical devices and methods that utilize gigantic photon-photon interactions. Another object is to provide such optical devices and methods that operate with single or few photons.
Various advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there is illustrated and described specific examples in accordance with the invention.